It has been known that, in the injection molding machine, the injection speed of a molten resin largely influences properties (precise and stable moldability or the like) of moldings. Hence it is necessary to control properly the injection speed of the molten resin to be filled into a mold.
In order to control the injection speed as described above, in the so-called screw-type injection molding machine, the advancing speed of a screw which applies pressure to the molten resin charged in a barrel is properly changed, and thereby the injection speed of the molten resin to be filled in the mold is controlled.
Heretofore, in order to change the advancing speed of the screw as described above, a flow control valve is installed in an oil pipe which supplies working oil to a driving device (a hydraulic cylinder, an injection ram or the like) for advancing the screw, and the flow rate of the hydraulic oil is controlled by this flow control valve.
Now, an example of such conventional control method will be described. When, for example, the multistages of the injection speed in one injection process are set in order to improve properties of moldings, a position detector for detecting the position of the screw, a setting device of the switching position of the screw speed and a setting device of the screw speed (the injection speed) are used. Control is performed so that the degree of opening of the flow control valve corresponds to the degree of opening for a preset flow rate at a preset screw position, thereby the screw speed is controlled in multiple stages. It is to be noted that such control methods have been disclosed, for example, in Japanese Patent Provisional (Kokai) Publication No. 48-95458 (1973) and Japanese Patent Provisional (Kokai) Publication No. 59-64337 (1984).
In the conventional control method described above, the injection speed is controlled merely by controlling the travel speed of the screw. Hence, the travel speed of the screw does not always correspond to the actual injection speed of the molten resin injected through the nozzle.
This is caused, for example, by the following phenomenon. When the screw presses the molten resin during the filling process, the pressure of the molten resin within the barrel formed in the front end of the screw is increased. If the forward movement of the screw is halted under this condition, the injection speed of the molten resin is expected to become zero. Actually, however, the compressed molten resin within the barrel formed in the front end of the screw flows out through the injection nozzle into the mold until its pressure is reduced down to the pressure corresponding to the loading pressure within the mold, and as a result, the actual injection speed does not become zero immediately when the movement of the screw is halted.
Another example, in which the actual injection speed also differs from the injection speed set by the travel speed of the screw, is as follows. Contrary to the first example, when the screw is rapidly accelerated from the halting state, it is necessary for the pressure of the molten resin to be sufficiently increased until the actual injection speed corresponds to the screw speed. Hence, there occurs a delay in response corresponding to the elastic compression time of the molten resin, and it will be understood that the actual injection speed differs from the injection speed set by the travel speed of the screw during that time interval. The reason why there is a difference between the injection speed of the actually injected molten resin and the injection speed set by the moving speed of the screw is considered to be due to elasticity of the molten resin.
As described above, in the conventional control method, the actual injection speed of the molten resin can not be exactly controlled no matter how exactly the screw speed is controlled. Hence, repeatability of the injection time and stability of the injection speed are destroyed, and it is difficult to improve moldability for greater accuracy and stability.
It is an object of the present invention to provide; taking into account of the above-described circumstances, a control method of an injection molding machine which secures as much as possible repeatability of the injection time and stability of the injection speed and thereby improves moldability for greater accuracy and stability.